Conventional techniques for making and separating semiconductor devices include depositing layers to form numerous semiconductor devices on a wafer substrate and then using mechanical techniques to separate the individual devices. The separation is typically performed by dicing or scribing the substrate to separate the individual devices. Dicing is typically done with a diamond saw, diamond scriber or laser, which is a time consuming process performed by very expensive machines. Several well-known problems exist with conventional techniques including process yield issues, device performance issues and processing cost issues.
FIGS. 1A-C depict conventional semiconductor device separation techniques according to the prior art. FIG. 1A depicts a separation technique by mechanical dicing. FIG. 1B depicts a separation technique by mechanical scribing. FIG. 1C depicts a separation technique by laser scribing.
1. Process Yield Issues
According to conventional mechanical device separation methods, such as dicing and scribing methods, and a laser scribing method, each individual device is separated by cutting along a grid line, or street line, between the devices with the selected method. This is a slow process since each of the street lines is cut one at a time and sequentially
Process yield issues become more significant for semiconductor devices having hard substrate materials, such as GaN on sapphire or GaN on SiC materials. Furthermore, the separation yield is greatly affected by any cracks or defects created by substrate grinding and polishing. If the cutting lines pass through defective areas, the result is very low device separation yield.
As a result, device separation is known to be the most tedious and low yield process among the entire semiconductor device fabrication processes. In practical terms, the back-end process yield for the GaN-based semiconductor fabrication is known to be as low as less than 50%, while the front-end fabrication process yield is typically in the range of above 90%.
2. Device Performance Issues
Due to the physical abrasive action of dicing and scribing, the device performance after device separation may be significantly deteriorated. For example, the LED device side wall where the light emits may become damaged due to abrasive cutting action during device separation, which is the main cause of light output reduction after device separation.
In the case of laser scribing, the device separation is accomplished by melting the substrate material with a high intensity laser beam. As a result, the melted substrate material often accumulates on the side wall of the device, which results in lowering light output of the LED device as well.
3. Processing Cost Issues
The average die separation processing time for GaN/sapphire LED device having approximately 10,000˜12,000 devices per wafer is approximately 40 min to 1 hour with the conventional separation methods. This means that one device separation machine can handle only 24 to 36 wafers per day (700˜1,000 wafers/month) if the machine operates 24 hours/day. In order to achieve a commercially desirable factory output, many machines and significant capital equipment investment is needed.
In addition, the diamond cutting wheels for dicing machine and diamond tips for the scribing machine are very expensive consumable parts, hence there are significant consumable part cost involve with the conventional die separation processes.
In the case of laser scribing, the major consumable part is the laser source. In order to maintain constant laser beam energy, the laser source gas must be recharged regularly. The laser source is the one of the most expensive components in the laser scribing system.
What is needed is an improved technique for fabricating and then separating the devices that is reliable, economical and promotes consistently high device characteristics.